NONLINEAR ANALYSIS BY STRENGTH EVALUATION AND FABRICATION METHOD OF THINWALLED AIRFRAMES

摘要

The simplest stability analysis of thernairframe panels of up-to-date passenger andrntransport aircrafts with the cells of the skin withrnthe size of 400÷500 mm into the length andrn200÷300 mm into the width, with the thicknessrnfrom 0.8 to 1.5 mm, depending on aggregate,rnshows that the breaking stresses of their localrnstability are located in the region of 10÷20rnmPa. But it is assumed at designing, andrnpractice confirms it, that the panels oftenrnexperience membrane stress above 100 mPа.rnBut it is assumed at designing, and practicernconfirms it, that the panels often experiencernmembrane stresses of more than 100 mPа. As arnresult, significant parts of the airframe at somernloadings may be in a state of nonlinearrndeformation far beyond the stability of the skin.rnIn some cases, geometric nonlinearity can bernalso accompanied by nonlinearity of materialrndeformation. Of course the aforesaid is not arndiscovery, but in the airframe design practicernstill prevails the linear analysis, not taking intornaccount the deformation beyond stability limits.rnNonlinear analysis remains still a challenge, butrnsince the linear analysis does not appearrnsufficiently correct procedure of strengthrnevaluation, solution of this problem is extremelyrnactual for practice.rnThe decision of the problem is restrainedrnby inadequate for practice theoretical supportrnand insufficient automation of the systems ofrnnonlinear equations solution algorithms as wellrnas large sizes of these systems. The latter isrncaused by both the sizes and structuralrncomplexity and by the quality of the availablernfinite-element base, since the finite elementrnmethod (FEM) is usually used for the solution ofrnpractical problems. And the quality of finiternelements (FE) is determined by convergence.rnDespite the large number of studies undertakenrnso far to create efficient FE, in the manuals forrnthe use of practically all known modernrnprograms of the finite-element strength analysisrnquite strict requirements for the ratio of the FErnplates and shells sizes are still contained. Noncompliancernleads to the sharp loss in thernaccuracy of results. However, the observance ofrnrequirements is accompanied by the increase inrnthe systems of equations and not less sharprnincrease of calculation effort, which becomes anrnobstacle to the solution of many practicallyrnimportant problems. On the way to improve thernconvergence it was possible to create FE for thernsimulation of plates and shells, and also beamtypernelements to a considerable degree freernfrom the typical limitations to the ratio ofrndimensions and characterizing by the improvedrnconvergence when solving both the linear andrnnonlinear problems. Elements in the process ofrnsolution of systems have six degrees of freedomrnat the nodes (three displacements and threernrotation angles) and are able to take momentsrnabout the surface normal at the nodes.